Conventionally, as a lighting device for turning on the solid state light emitting elements, there is known a lighting device which has a control switch for supplying a constant current to the solid state light emitting elements, and supplies to the control switch a dual signal obtained by combining a high frequency drive pulse signal and a low frequency burst signal.
For example, in a power feeding assembly disclosed in Japanese Patent Application Publication No. 2006-511078, a dual signal obtained by performing an AND operation on a high frequency drive pulse signal and a low frequency PWM signal is supplied as a drive signal of a control switch. In the power feeding assembly, an average current flowing through the solid state light emitting elements is changed by changing a duty ratio of the low frequency PWM signal, and the solid state light emitting elements are turned on at a desired dimming level.
In this type of the lighting device, a dimmer being widely used for dimming of inverter type fluorescent lamps is used as a signal source outputting a PWM signal at a low frequency (about 1 kHz) since it can be supplied at a low cost. However, since a response speed of the solid state light emitting elements is faster than that of the fluorescent lamps, particularly, in case that the dimming level is low, there is a problem that a change in light output can be realized visually when the duty ratio of the PWM signal is changed.
Thus, there has also been proposed an LED lighting device including a dimming signal conversion circuit which operates by receiving a low frequency PWM signal outputted from this type of the dimmer, and converts it into a PWM signal with a variable pulse width in more multiple stages than the input PWM signal (see, e.g., Japanese Patent Application Publication No. 2010-198760). In such LED lighting device, the PWM signal is converted into the multi-stage PWM signal having more multiple stages by the dimming signal conversion circuit. Accordingly, while using a dimmer processing a small number of bits, it is possible to achieve a smooth change in dimming level as in case of using a dimmer processing a large number of bits.
Meanwhile, in the above-described lighting device, the drive signal of the control switch is an AND output of the high frequency drive pulse signal and the low frequency PWM signal. When a falling edge of the PWM signal is inputted while the control switch is in an ON state, the drive signal of the control switch becomes a low level. Accordingly, the ON period of the control switch is changed by a change in the low frequency PWM signal, and the current flowing through the solid state light emitting elements is changed, thereby changing the light output. Further, in the OFF period of the control switch, a regenerative current of an inductor included in the lighting device flows through the solid state light emitting elements. Thus, even if the PWM signal is changed during the OFF period of the control switch, the current flowing through the solid state light emitting elements does not change.
Therefore, as in the LED lighting device disclosed in Japanese Patent Application Publication No. 2010-198760, even if the duty ratio is continuously changed by artificially increasing the number of bits in the PWM signal, there is a problem such that the change in current flowing through the solid state light emitting elements is delayed and the light output is changed in a step shape (see FIG. 18). Particularly, in case that the dimming level is low, since a rate of change in light output is large, there is a problem that the change in light output is easily noticeable.
Further, when the light output of the solid state light emitting elements is seen through video equipment such as video cameras, a flicker that interferes with a specific frequency of the video equipment is visually seen. For that reason, it is necessary to set the frequency of the low frequency PWM signal to be higher than a predetermined value. Furthermore, when the frequency of the low frequency PWM signal increases, the light output for one cycle of the high frequency drive pulse signal of the control switch becomes larger. Therefore, it is necessary to further increase the frequency of the high frequency drive pulse. However, in case of increasing the frequency of the high frequency drive pulse, since the switching loss increases or the parts become expensive, it is difficult to significantly increase the frequency.